Design, construction, operation and maintenance specifications for vegetated filter strips
Link to Work Order work plan describing scope of work for updates to this page: File:Filter strips as pretreatment practices work plan.docx
Vegetated filter strips are a pretreatment Best Management Practice (BMP) designed to remove solids from stormwater runoff. The vegetation can consist of natural and established vegetation communities and can range from turf grass to woody species with native grasses and shrubs. Because of the range of suitable vegetation communities, vegetated filter strips can be easily incorporated into landscaping plans; in doing so, they can accent adjacent natural areas or provide visual buffers within developed areas. They are best suited for treating runoff from roads, parking lots and roof downspouts.
Their primary function is to slow runoff velocities and allow sediment in the runoff to settle or be filtered by the vegetation. By slowing runoff velocities, they help to attenuate flow and create a longer time of concentration. Filter strips do not significantly reduce runoff volume, but there are minor losses due to infiltration and depression storage.
Filter strips are most effective if they receive sheet flow and the flow remains uniformly distributed across the filter strip. Channelized flow significantly reduces the effectiveness of a filter strip because flow velocities are minimally reduced and the effects of filtration through plant stems and accumulated thatch on the ground surface is lost.
Contents
Special receiving waters suitability
Design restrictions for filtration BMPs hold true for vegetated filter strips specifically. While the design of filter strips can utilize a variety of vegetation communities, native vegetation with robust root systems should be used immediately adjacent to water bodies to provide additional bank stabilization and provide natural cover for habitat.
Design restrictions for special waters
Design considerations
There are five critical components for an effective vegetative filter strip design:
- contributing drainage area;
- type of vegetation;
- filter strip length;
- filter strip slope; and
- filter strip soils.
Vegetated filter strips are most effective for storm events up to the 1- to 2-year event. The effectiveness of filter strips is reduced for flow depths that exceed 1 inch and flow velocities that exceed 0.5 feet per second.
The flow length leading to the vegetated filter strip is a primary design factor because if the flow length is too long, there is a greater chance that the sheet flow will become concentrated flow and require additional design to re-establish sheet flow entering the vegetated filter strip. The flow length into the vegetated filter strip should be limited to 75 feet from impervious surfaces and 150 feet from pervious surfaces.
Vegetated filter strips are most effective if the vegetation is healthy and dense. Grasses are most effective in shorter filter strips while woody species may be suitable for longer filter strips.
A properly designed filter strip length is dependent on the slope, the underlying soils, and the type of vegetation to be used. Filter strips that are too short will have reduced effectiveness. Filter strips with steeper slopes will require more length to be equally effective as a filter strip with a lower slope; slopes less than 5 percent are most effective. The current state of Minnesota guidelines are consistent with other published guidelines.
Guidelines for filter strip pre-treatment sizing
Link to this table
| Parameter | Impervious Parking Lots | Residential Lawns | ||||||
|---|---|---|---|---|---|---|---|---|
| Maximum Inflow Approach Length (ft) | ||||||||
| Filter Strip Slope | =<2% | >2% | =<2% | 2% | =<2% | 2% | =<2% | 2% |
| Filter Strip Minimum Length | 10' | 15' | 20' | 25' | 10' | 12' | 15' | 18' |
Construction
One significant benefit of vegetated filter strips is that construction is relatively inexpensive since it is limited to grading and vegetation establishment. Maintaining sheet flow in the filter strip is critical to its effectiveness; therefore, construction must result in an evenly graded site. Care should also be taken to avoid excessive compaction in the filter strip area to maintain healthy soils for a healthy vegetation community and to preserve the infiltration capacity of the soils.
Maintenance
If designed properly and if they receive sheet flow, vegetated filter strips require minimal maintenance. More maintenance is required of vegetated filter strips that receive channelized flow from the contributing watershed or concentrated flow from curb cuts. Systems that redistribute concentrated or channelized flow into sheet flow may require periodic maintenance to ensure the redistribution system maintains its design function. Depending on the contributing watershed characteristics, common maintenance needs include periodic removal of accumulated sediment and debris (particularly at the upstream end of the filter strip) and monitoring to ensure channels or preferential flow paths have not developed. Contributing watersheds with high sediment concentrations require more frequent maintenance in the vegetated filter strip to manage sediment build-up. Maintenance may also include mowing and trimming or burning the plant community depending on specific needs of the vegetation.